1. Field of the Invention
This invention relates to a semiconductor device having a novel electrical plasticity used for information processing through a neural network and further to the control method thereof.
The plastic device herein mentioned refers to a device which shows non-linear electrical characteristics, i.e. those in electrical conductivity and capacity, depending on the energy of input, more specifically on its amount or change with time.
2. Description of the Prior Art
The information processing through a neural network, although a fine information processing performed in brains of living things, has so far been accompanied by a problem in that there is no quality electronic device which functions like a synapse. This problem has confronted future information processing.
The neurochips having already been proposed can be classified into two types, semiconductor devices and optical devices. Although the neurochips using semiconductor devices have greater feasibility, they include none having appropriate characteristics for electronic devices having the plastic function. The result is that bipolar devices or CMOSs using a silicon semiconductor or the like are being improved as the devices performing analog operations for the development of a neural network.
On the other hand, the devices which are discussed for serving like a synapse to give the neural network a plastic function include the field-effect transistor (FET), transconductance amplifier (OTA), capacitor array, switched register, and so forth. Among them, the FET has excellent features such as low power and high gain, and those such as floating gates for EPROMs and MNOSs are attracting the eyes of those skilled in the art.
As for electrically conductive polymers, it is common knowledge that they are composed of polymers having large-extended conjugated .pi. electron systems such as polyacetylene, polypyrrole, polythiophene, polyaniline, polyacene, and so forth, and contain electron donors (e.g. metal ions) or electron acceptors (anions such as Lewis acid or protonic acid) as a dopant, thus showing a high electrical conductivity. The resultant dopant, however, has had a large drawback that it will be diffused in a polymer matrix due to an electric field, thereby causing its conductivity to be varied.
Molecule-based transistors that have made use of the above drawback in turn are disclosed in Journal of American Chemical Society, 106, p.5375, 1984 edition by M.S. Wrighton et al., and ibid. 109, p.5526, 1987 edition by M.S. Wrighton.
Furthermore, an electrically conductive variable device intended for a plastic device is disclosed in Japanese Patent Laid-Open No. 63-200396.
However, the conventional devices described above have been unsatisfactory in that they have neither appropriate characteristics for an electronic device nor those for a plastic device.
It is therefore the primary object of this invention to provide an electrically plastic device having high-gain high-performance characteristics using a novel electrical control device. Another object of this invention is to provide a control method for this electrically plastic device.
This invention provides an electrically plastic device in which an electron conjugated polymeric semiconductor layer containing a dopant or a multilayer structure comprising an electron conjugated polymeric semiconductor layer and a mobile dopant retaining layer is formed between or across a pair of electrodes, and at least one gate electrode is provided between the pair of electrodes with an insulating layer or a high-resistance layer interposed along the electron conjugated polymeric semiconductor layer, wherein the dopant distribution in the electron conjugated polymeric semiconductor layer or mobile dopant retaining layer is controlled through the gate electrode, thereby controlling the conductivity of the electron conjugated polymeric semiconductor layer.
One of the control methods for electrically plastic devices in accordance with this invention is carried out in such a way that a positive or negative pulse voltage is applied to the gate electrode to control the dopant distribution in the electron conjugated polymeric semiconductor layer, thereby varying the conductivity thereof, which is used as a learning or reset operation to make the gate electrode at ground potential so that a pair of electrodes (source and drain) are driven into conduction. Another control method for the same is such that the gate electrode is short-circuited to one of the pair of electrodes (source) and a positive or negative pulse voltage is applied between the pair of electrodes so as to control the dopant distribution in the electron conjugated polymeric semiconductor layer, thereby varying the conductivity thereof, which is used as a learning or a reset operation so that the pair of electrodes are driven into conduction with a voltage lower than the pulse voltage.
The electrically plastic device of this invention can control the dopant distribution in the electron conjugated polymeric semiconductor layer or dopant retaining layer through the gate voltage, thereby varying the electrical conductivity of the electron conjugated polymeric semiconductor layer, capable of amplifying the current across a pair of electrodes (equivalent to source and drain of FET) to a large extent, allowing them to be switched.
This electrically plastic device, whose operation is equivalent to the FET operation of a dopant mobile semiconductor, has excellent memory type non-linear electrical characteristics practically usable as a plastic device. More precisely, in the electrically plastic device of this invention, when a voltage is applied to the gate electrode, it will be subjected to a substantial change in channel impedance as the number of carriers due to field effect is affected by the time-varying dopant concentration. This movement of the dopant is of a memory type and would never be done without gate electric field. Accordingly, when a network is formed of the plastic devices in accordance with this invention, a superior neural network with a high S/N ratio may be formed by virtue of a high change ratio in its channel impedance.